HIV-1 is the name given to a group of highly related viruses which have been identified as the primary etiologic agent of the acquired immunodeficiency syndrome (AIDS) and AIDS related condition (ARC) in humans. HIV-1, also known as HTLV-III, LAV and ARV, is a major worldwide health problem.
HIV is a relatively complex retrovirus containing at least seven genes. The viral structural genes designated gag, pol and env respectively code for the viral core proteins, reverse transcriptase, and the viral glycoproteins of the viral envelope. The other HIV genes are accessory genes involved in vital replication. The gag and env genes encode polyproteins, i.e., the proteins synthesized from each of these genes are post-translationally cleaved into several smaller proteins. Previous studies have shown that the proteins coded by the gag and especially the env regions of the HIV genome are immunologically important, since antibodies to the products of the gag and env genes are found in the sera of patients infected with HIV.
The env gene encodes a glycoprotein (gp160) with an apparent molecular weight (Mw) of about 160,000 Daltons which is post-translationally cleaved into two glycoproteins, gp120 and gp41, of Mw 120,000 and 41,000 Daltons, respectively. Glycoprotein gp120 is the external protein of the viral envelope, while gp41 is a transmembrane protein. The gp120 protein associates noncovalently with gp41 such that gp120 is exposed on the cell surface. Both gp120 and gp41 are immunogenic, with antibodies to the proteins readily detectable in sera obtained from patients infected with HIV but who are asymptomatic, and ARC and AIDS patients.
The HIV coat protein gp120 binds to the T cell surface protein CD4, also known as the HIV receptor protein. The gp120 protein is crucial to HIV infection, because gp120 binds to CD4 with high affinity to effect HIV entry into the cell. Cells infected by HIV express a cell surface form of gp120 and also shed a soluble form of gp120 protein from their surface. This extracellular, soluble gp120 binds to CD4 on uninfected cells with high affinity and appears to induce cell death by an unknown mechanism. It is characteristic of HIV infection that infected cells may not lyse, consequently cells expressing gp120 may survive for extended periods of time, acting as reservoirs of the virus and gp120.
HIV infection is largely dependent on gp120 binding to CD4 as underscored by the finding that soluble recombinant CD4 lacking its transmembrane and cytosolic sequences can block HIV infectivity and soluble gp120 mediated cell killing. The soluble CD4 also binds to HIV infected cells and soluble gp120. Smith et al., "Blocking of HIV-1 Infectivity by a Soluble, Secreted Form of the CD4 Antigen", Science, 238: 1704-1706 (1987). The use of soluble CD4 as a therapeutic agent is expensive, however, and there are problems with delivery and stability. Furthermore, CD4 treatment can result in the formation of antibodies specific to CD4 with subsequent autoimmune disease. Weber, "Blocks on the Viral Exit", Nature, 345: 573-574 (1990).
HIV infection is also thought to occur by cell-to-cell transmission. This form of infection does not rely on extracellular HIV binding to CD4, and cannot be prevented by agents such as soluble CD4. In order to effect cell-to-cell transmission, an infected cell forms contacts termed syncytia with a non-infected cell thus enabling direct transmission of the virus. Syncytia are visible under a light microscope and are indicators of HIV infection.
Since HIV-1 was first identified as the etiologic agent of AIDS, substantial progress has been made in studies on the virus per se, mechanisms by which the virus causes disease and in the development of diagnostic tests to detect exposure to the virus or infection. Progress in HIV vaccines and therapy has been slow due to the heterogeneous nature of the virus and the lack of suitable animal models. See, e.g., Martin, "Fast-Acting Slow Viruses", Nature, 345: 572-573 (1990).
While a variety of approaches have been taken to formulate pharmaceutical agents suitable for AIDS therapy, many if not all of the drugs create serious side effects which greatly limit their usefulness as therapeutic agents.
One drug target is the HIV protease crucial to virus development. HIV protease is an aspattic protease and hence can be inhibited by agents such as H-261 (tBoc-His-Pro-Phe-His-Leu.psi.[CHOH-CH.sub.2 ]Val-Ile-His) (Seq. I.D. No. 2) and acetyl-pepstatin. Richards, "Inhibition of the Aspattic Proteinase from HIV-2", FEBS Lett., 253: 214-216 (1989). Unfortunately, inhibitors of aspattic proteases are non-selective and therefore toxic when used in vivo. In addition, several peptide analogues have been found to inhibit HIV protease. Meek et al., "Inhibition of HIV-1 Protease in Infected T-lymphocytes by Synthetic Peptide Analogues", Nature, 343: 90-92 (1990). Recently a two-fold symmetric inhibitor of HIV protease was described. Erickson et al., "Design, Activity, and 2.8 .ANG. Crystal Structure of a C.sub.2 Symmetric Inhibitor Complexed to HIV-1 Protease", Science, 249: 527-533 (1990). This symmetric inhibitor has a selective activity against HIV-1 protease, it is about 10,000 fold more potent against HIV-1 protease than against related cellular enzymes.
The HIV reverse transcriptase is likewise necessary for effective HIV infection and thus; drugs are being sought which inhibit the enzyme's activity. Several nucleoside derivatives have been found to inhibit HIV reverse transcriptase. Foremost among these drugs is azidothymidine (AZT, Zidovidine.RTM.). AZT causes serious side effects, however, such that many patients cannot tolerate its administration. Other nucleoside analogues that inhibit HIV, reverse transcriptase have been found to cause even more serious side effects then does AZT.
Several agents have been found to inhibit binding of HIV to T cells. For instance, a pentapeptide and an octapeptide derived from vasoactive intestinal peptide (VIP) have been found to inhibit HIV infection. Moore et al., "In vivo Depression of Lymphocyte Traffic in Sheep by VIP and HIV (AIDS)--Related Peptides", Immunopharmacol., 16: 181-189 (1988). Unfortunately, these peptides have the side effect of mimicking the immunosuppression of HIV infection by inhibiting proliferation of T4 cells and are thus useless for therapy.
Recently, N-carbomethoxycarbonyl-prolylphenylalanyl benzyl ester (CPF), a derivative of the dipeptide prolylphenylalanine, was shown to inhibit HIV-1 infection in vitro. CPF interacts with gp120 and blocks the binding of gp120 to CD4. Finberg et al., "Prevention of HIV-1 Infection and Preservation of CD4 Function by the Binding of CPF's to gp120", Science, 249: 287-291 (1990).
Small peptides have been used in the treatment of measles virus and herpes virus infections. A series of carbobenzoxy (Z) peptides, including Z-D-Pro-D-Phe have been shown to inhibit measles and herpes viruses. Miller et al., "Antiviral Activity of Carbobenzoxy Di- and Tripeptides on Measles Virus", Appl. Microbiol., 16: 1489-1496 (1968); and Nicolaides et al. "Potential Antiviral Agents. Carbobenzoxy Di- and Tripeptides Active Against Measles and Herpes Viruses", J. Med. Chem., 11: 74-79 (1968). These compounds interact with the target cell and not with the viral protein.
It would be useful in the therapy and prevention of AIDS to have a specific, selective anti-HIV therapeutic agent that causes few, if any, side effects.